The present invention generally relates to a heat roller type fixing device for use in an electrophotographic copier, facsimile apparatus or similar image forming equipment and, more particularly, to a method of controlling the surface temperature of a fixing roller which is installed in such a fixing device.
In an image forming equipment of the kind described, a heat roller type or similar fixing device is installed for fixing by fusion a toner image which has been transferred from an image carrier to a paper sheet. A heat roller type fixing device has a fixing roller in which a heating element or heater is built in, a pressing roller held in pressing contact with the fixing roller, and a thermistor pressed against the surface of the fixing roller by a predetermined force to sense the surface temperature of the latter. The heater is turned on and off on the basis of the sensed surface temperature of the fixing roller, whereby the surface temperature is constantly controlled to a predetermined range. However, when the thermistor is pressed against the fixing roller by a less intense force than expected or spaced apart from the roller surface, it is apt to occur that the actual surface temperature of the roller is higher than the predetermined range despite that the thermistor output is indicative of a temperature lying in that range. Such an occurrence may cause a paper sheet to curl more than necessary or, in the worst case, overheat and thereby damage the fixing device. This kind of unusual condition is brought about by a failure of the thermistor element itself also. In any case, it is impossible to maintain the surface temperature of the fixing roller in the predetermined range and therefore to attain a desired fixed image.